Inorganic crystals such as quartz, rochelle salt, or barium titanate have been known for a long time as piezoelectric materials, and in recent years some vital substances or synthetic polymers have become known to have piezoelectric properties. As living tissues having piezoelectric properties, there have been known bones, skins, blood vessels, muscles, hairs, ivories, silk yarns, bamboos, timbers, and the like, and it is also considered that most proteins have piezoelectric properties. Piezoelectric properties are also observed in uniaxially stretched films of polymers of simple amino acids such as polybenzyl glutamate or polymethyl glutamate which are synthetic polymers.
On the other hand, when a direct current high voltage is applied to a polymeric film at relatively gigh temperatures and the film is then cooled, it becomes the so-called electret. It has long been known that electrets have piezoelectric properties. Recently, with the development of polymeric electrets, the piezoelectric properties of various electrets have been examined, and it has been found that films of polar polymers such as polyvinylidene fluoride (to be referred to as PVDF), polyvinyl fluoride, or polyvinyl chloride especially have high piezoelectric properties. In particular, PVDF has been found to have high piezoelectric properties. For example, PVDF film is uniaxially stretched at 120.degree.-150.degree. C to several times the original length, and formed into an electret in an electric field of about 300 KV/cm at 50.degree.-90.degree. C. This results in the formation of a piezoelectric PVDF film having a piezoelectric constant d.sub.31 of a maximum of 2 .times. 10.sup..sup.-7 c.g.s.e.s.u. (the stretching direction being X axis).